1. Field
The present invention relates to the field of microwave devices.
More specifically, the present invention relates to the field of devices comprising several individual microwave sources supplied from a common generator.
2. Description of Related Art
The present invention may especially find applications in the production of a plasma from a given number of individual plasma sources supplied with microwave power from a single power generator.
These individual sources may either be independent in the same chamber (the objective being, for example, to overcome the physical or technological limits on the maximum microwave power that it is possible to apply to a single plasma source) or be distributed in the same chamber so as to allow the extension of scale needed for an intended application. In general, the fields of application of multiple plasma sources may cover not only all the fields already covered by the use of single plasma sources, but also novel fields that cannot be envisioned with unitary sources (for example for reasons of uniformity, rates, etc.).
The invention relates to all microwave plasmas and discharges, whatever the pressure range, the microwave frequency, the nature or the configuration of the microwave applicator and the presence or absence of a magnetic field.
However, the invention is not limited to the field of plasmas. It may, for example, also be applied for bonding, drying or curing operations using multiple stations and more generally for any operation in which the impedance of the system may vary over time from one station to another.
The microwave field has already been the subject of extensive research.
Several proposals have already been made to supply several individual sources from a common generator.
To divide the microwave power delivered by a single generator, it is possible to use cascades of 3 dB couplers (division by 2) which are formed, for example, from rectangular waveguides. This solution, although often requiring a very large amount of space, makes it possible to produce power divisions by N=2k, where k represents the number of successive levels of the cascade. Thus, the microwave power may be divided by 2, 4, 8, 16, 32, etc. A matched coax/waveguide transition at the end of each waveguide furthermore allows the microwave power to be transported by means of coaxial cables fitted with standard connectors.
Another widely used solution is to take off microwave power either into a cavity, or into a waveguide, or into a ring resonator, in which cavity, waveguide or ring resonator standing waves are created, by antennas placed at the electric field antinodes (regions of maximum electric field. This solution assumes in general that each individual plasma source behaves as a matched impedance, in other words that it absorbs all of the microwave power taken off. With such a device, it is then possible to deliver a predetermined microwave power to each individual source.
However, the devices proposed hitherto are not completely satisfactory.
One of the difficulties of dividing microwave power for the purpose of supplying plasma sources is that, as a general rule, a plasma source does not behave as a matched load. This is because the impedance taken back to the input of a plasma source, resulting from the combination of the input impedance of the applicator and of the impedance of the plasma taken back to this input, does not generally correspond to a matched load, that is to say a purely resistive load equal to the characteristic impedance of the microwave supply line. On the contrary, one may be faced, depending of the type of discharge, the discharge conditions and the absorbed power, with complex impedance values at the input of the plasma source, values which vary from zero to infinity.
In the case of several plasma sources fed by the same microwave generator, there is also the problem of the influence of the impedance of a source on all of the other sources in the absence of sufficient decoupling (typically greater than 20 dB) between the supply lines for the various microwave sources.
Thus, immediately after ignition, the input impedance of the source is generally much higher than that corresponding when the discharge is in the steady state. Apart from this variation in impedance for a given source at the moment of ignition, the power distribution is also affected by the various plasma sources not being ignited simultaneously. Consequently, when turning on a number of plasma sources one is necessarily confronted with significant imbalances in the power transmitted to the plasma sources and with the introduction of considerable reflected power into the circuit.
These imbalances, which cause very high reflected power levels, may prevent the plasma from being turned on in the sources requiring a minimum plasma density, and hence a minimum transmitted power, such as for example in surface wave plasmas.
Another difficulty involving impedance imbalance arises in the case of plasmas whose plasma density is, on the contrary, limited to an upper value, for example the critical density, as in plasmas using distributed electron cyclotron resonance. In this case, the entire incident power greater than the value ensuring the critical density is reflected at the input of the source and sent back into the microwave distribution circuit.
Furthermore, impedance imbalances may also be encountered during operation, for example should one of the sources fail, or after an intentional or unintentional variation in the operating conditions (composition of the gas, flow rate, pressure, density of the plasma, radio frequency bias, etc.) during multisequence processes.
Finally, in the case of several plasma sources operating in the same chamber, the interference between applicators also results in reflected power levels which may disturb the desired power distribution.
Thus, the conventional solutions for microwave power division are either excessively bulky (cascades of 3 dB couplers) or allow only division by prescribed numbers n=2k, or require a matched impedance, which is not the case with a plasma source.
The objective of the present invention is to improve the microwave systems comprising several individual sources fed from a common generator, so as to eliminate the drawbacks of the prior art.
This objective is achieved within the context of the present invention by means of a system comprising:
a microwave generator;
a rectangular waveguide coupled to the generator, matched in order to operate in the fundamental mode (H10) or in the transverse electric mode (TE10), and combined with means ensuring standing wave conditions;
a plurality of power output ports placed in the waveguide in the regions of maximum amplitude of one of the components of the electromagnetic field in order to provide power division for the generator, the power output ports being adjusted in such a way that the sum of their reduced admittances brought back to the input of the divider formed by the rectangular waveguide is unitary and
a plurality of sources which are coupled respectively to an output port by the agency
of an isolator means ensuring power transmission from the output port to the source, without being reflected back to the output port, and
of a device for matching the impedance of each source, said device being located downstream of the isolator means, between the latter and the associated source.